Lithium-ion battery is known as a high energy density battery with high output voltage in average and high output power. It has low self-discharge, no memory effect, wide working temperature range (from −20° C. to 60° C.), excellent cycling performance, high charging and discharging efficiency, and long life. The lithium-ion battery is a green battery which does not pollute the environment. Increasingly, the lithium-ion battery is being used in the electric vehicle, reducing cost for consumers and representing a development trend in future.
However, compared to traditional applications of the lithium-ion battery in cell phones, laptop computers and other small appliances, the electric vehicle requires the lithium-ion battery with a higher requirement. The electric vehicle not only requires the power supply to have a higher stability and security, but also requires the power supply to have a longer continuous duration. A fluctuation of load on the electric vehicle is also very large. Therefore, the power supply for the electric vehicle requires a higher charging and discharging performance, as well as a higher capacity.
Existing battery design fails to meet both requirements of high capacity performance and high power performance at the same time. For example, the lithium-ion secondary battery, as a typical high capacity battery, is designed to increase its capacity so as to maximize the mileage for the electric vehicle. The battery packs of the lithium-ion secondary battery can be discharging for a long and continuous time under 0.2 C in applications with low-load. However, this type of battery cannot meet the driving requirement, especially the acceleration requirement for the electric vehicle. Therefore, the above-mentioned lithium-ion secondary battery with high capacity cannot fit in applications with high-load such as in the electric vehicle. In order to meet the acceleration requirement for the electric vehicle with high-load, super capacitors with high power are proposed to be set in a discharging circuit of the battery.
Chinese Issue No. CN101489824B, entitled “vehicle power control device”, discloses a proposal of combining high capacity battery with capacitor, so as to meet the high-load power requirement during acceleration of a vehicle. In the power control device, a high capacity battery, a capacitor and a voltage booster are connected in sequence. The capacitor has the characteristic of a fast charging and discharging performance under a large current, and the power of the battery is boosted by the voltage booster to finally drive an electric motor of a vehicle. Due to the limited capacity of the super capacitor, the power control device can only be used under special situations to meet the high-load requirement within a short time. The battery fails to output high power continuously for several or dozens of minutes. Also, the battery neither improve a charge rate, nor extend a driving mileage.
Chinese Publication No. CN101005148A, entitled “hybrid power supply device”, discloses a proposal of connecting a high-capacity battery in parallel with a high-power battery to obtain a hybrid power supply, wherein the high-capacity battery and the high-power battery have different discharging properties. The discharging capacity of each cell in the high-capacity battery under 0.2 C is higher than that of each cell in the high-power battery under 0.2 C. Furthermore, a ratio of the discharging capacity of each cell in the high-power battery under 5 C to the discharging capacity of each cell in the high-power battery under 0.2 C, is higher than a ratio of the discharging capacity of each cell in the high-capacity battery under 5 C to the discharging capacity of each cell in the high-capacity battery under 0.2 C. Although the hybrid power supply can extend the mileage for a vehicle to a certain extent, the requirements of high-load and fast energy recovery cannot be satisfied, due to the discharging capacity of the high-power battery being 5 C, which is weaker than the discharging capacity of a super capacitor. The high-power battery is only used as a supplement for the high-capacity battery.